The present series of experiments are concerned with the physiological mechanisms underlying recovery of function from brain damage in young, mature and aged rats. One study examined the effects of single- or two-stage frontal pole removals on loss and recovery of function as the animals progressed from young adults to aged subjects. Initially, neither the one or two-stage operations produced a deficit but as the rats were tested at 6 month intervals, impairments on several behavioral tasks began to emerge and grew worse as the animals grew older. Another series of experiments, now in progress, have been designed to determine whether various kinds of pre- or post-operative enrichment experiences can ameliorate the consequences of brain damage inflicted either in young adults or senescent rats. Finally, a third project is exploring the behavioral and anatomical consequences of Nerve Growth Factor administration in mature and aged subjects.